A typical stent is a cylindrically shaped device, which holds open and sometimes expands a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been subjected to angioplasty or valvuloplasty.
A stent is typically composed of scaffolding that includes a pattern or network of interconnecting structural elements often referred to in the art as struts or bar arms. The scaffolding can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape.
In the case of a balloon expandable stent, the stent is mounted on a balloon connected to a catheter. A typical conventional method of mounting the stent on the balloon is a two-step process. First, the stent is compressed or crimped onto the balloon. Second, the compressed or crimped stent is retained or secured on the balloon. The retained stent should have a sufficiently small diameter so that it can be transported through the narrow passages of blood vessels. The stent must be secured on the balloon during delivery until it is deployed at an implant or treatment site within a vessel in the body of a patient. The stent is then expanded by inflating the balloon. “Delivery” refers to introducing and transporting the crimped stent through a bodily lumen to the treatment site in a vessel. “Deployment” corresponds to the expanding of the crimped stent within the lumen at the treatment site. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, inflating the stent at the treatment location, and removing the catheter from the lumen by deflating the balloon.
The stent should be firmly secured to the balloon to avoid detachment of the stent before it is delivered and deployed in the lumen of the patient. Detachment of a stent from the balloon during delivery and deployment can result in medical complications. A lost stent can act as an embolus that can create a thrombosis and require surgical intervention. For this reason, a stent must be securely attached to the catheter. Stent retention is greatly enhanced by protrusion of the balloon into the interstitial spaces or gaps between stent struts.
Stent retention for a polymer stent is especially challenging. Because polymers generally have lower strength than metals, a polymer stent requires wider struts than a metal stent to achieve the same mechanical strength. As a result, a polymer stent has smaller spaces or gaps between adjacent struts, making it more difficult for a balloon to protrude into the spaces to enhance stent retention. Additionally, a polymer stent may have a similar Tg as the balloon material so that there is a limitation on the use of high temperature to enhance balloon protrusion into stent gaps. When high temperature is used during a crimping process, the stent tends to return to its original dimension and lose its preferred mechanical properties.